The present invention relates to a new and improved excess-current protective switch.
Generally speaking, the inventive protective switch is of the type comprising a movable contact element which in the cut-on position is in engagement with a stationary contact element and is exposed to the action of a force displacing such contact element into a cut-off position. A locking device or mechanism is provided which can be released in the presence of an excess current and by means of which the movable contact element can be arrested or locked in the cut-on position.
Such excess-current protective switches are often employed instead of fuses, which only can be used once. They are installed in electrical switching installations working with medium and high voltages for protecting the same for instance against short-circuit or fault currents.
A known protective switch of the initially mentioned type is disclosed in U.S. Pat. No. 2,757,261 in conjunction with FIGS. 8 and 10, which correspond to FIGS. 4 and 5 of German Patent Application No. W 8851 VIII b/21c, published Sept. 6, 1956.
With prior art excess-current protective switches the spring-loaded, movable contact element is coupled with the locking device by means of a lever drive. The locking device, in turn, is provided with a pawl mechanism which is operatively associated with an electromagnet. Such electromagnet is provided with a movable armature cooperating with the pawl mechanism. The excitation or field coil of the movable armature consists of a few turns of a conductor connected in series with the movable contact element. If an excess current, for instance a short-circuit current flows through the conductor, the attraction force of the electromagnet upon the movable armature is sufficient to cause the armature to unlock the pawl mechanism. Thereafter the movable contact element is free to move under the action of the spring and to advance into the cut-off position.
The heretofore known protective switch has the disadvantage of containing numerous mechanically movable parts or elements and also numerous pivot pins, which is particularly unfavorable for switches of this type since, in the long run, such construction impairs the reliability of the protective switch. Moreover, such design is in direct opposition to the object of manufacturing such protective switches as simply and therefore as cheaply as possible, but without any loss in functional reliability.
Finally, with the state-of-the-art switch the excitation coil of the magnet, which always carries current in the cut-on position of the switch, continuously generates heat due to thermal losses which has to be dissipated. This heat dissipation can be accomplished by direct cooling measures or, as with the previously known switch, by arranging the elements of the switch in a relatively large internal chamber which allows for a sufficient circulation of the medium contained therein in order to obtain the required cooling effect.